As a result of environmental and other concerns, plastic containers, more specifically polyester and even more specifically polyethylene terephthalate (PET) containers are now being used more than ever to package numerous commodities previously supplied in glass containers. Manufacturers and fillers, as well as consumers, have recognized that PET containers are lightweight, inexpensive, recyclable and manufacturable in large quantities.
Blow-molded plastic containers have become commonplace in packaging numerous commodities. Studies have indicated that the configuration and overall aesthetic appearance of a blow-molded plastic container can affect consumer purchasing decisions. For example, a dented, distorted or otherwise unaesthetically pleasing container may provide the reason for some consumers to purchase a different brand of product which is packaged in a more aesthetically pleasing fashion.
While a container in its as-designed configuration may provide an appealing appearance when it is initially removed from a blow-molding machine, many forces act subsequently on, and alter, the as-designed shape from the time it is blow-molded to the time it is placed on a store shelf. Plastic containers are particularly susceptible to distortion since they are continually being re-designed in an effort to reduce the amount of plastic required to make the container. While this strategy realizes a savings with respect to material costs, the reduction in the amount of plastic can decrease container rigidity and structural integrity.
Manufacturers currently supply PET containers for various liquid commodities, such as juice and isotonic beverages. Suppliers often fill these liquid products into the containers while the liquid product is at an elevated temperature, typically between 155° F.-205° F. (68° C.-96° C.) and usually at approximately 185° F. (85° C.). When packaged in this manner, the hot temperature of the liquid commodity sterilizes the container at the time of filling. The bottling industry refers to this process as hot filling, and the containers designed to withstand the process as hot-fill or heat-set containers.
The hot filling process is acceptable for commodities having a high acid content, but not generally acceptable for non-high acid content commodities. Nonetheless, manufacturers and fillers of non-high acid content commodities desire to supply their commodities in PET containers as well.
For non-high acid content commodities, pasteurization and retort are the preferred sterilization processes. Pasteurization and retort both present an enormous challenge for manufactures of PET containers in that heat-set containers cannot withstand the temperature and time demands required of pasteurization and retort.
Pasteurization and retort are both processes for cooking or sterilizing the contents of a container after filling. Both processes include the heating of the contents of the container to a specified temperature, usually above approximately 155° F. (approximately 70° C.), for a specified length of time (20-60 minutes). Retort differs from pasteurization in that retort uses higher temperatures to sterilize the container and cook its contents. Retort also applies elevated air pressure externally to the container to counteract pressure inside the container. The pressure applied externally to the container is necessary because a hot water bath is often used and the overpressure keeps the water, as well as the liquid in the contents of the container, in liquid form, above their respective boiling point temperatures.
PET is a crystallizable polymer, meaning that it is available in an amorphous form or a semi-crystalline form. The ability of a PET container to maintain its material integrity relates to the percentage of the PET container in crystalline form, also known as the “crystallinity” of the PET container. The following equation defines the percentage of crystallinity as a volume fraction:
      %    ⁢                              ⁢                            ⁢    Crystallinity    =            (                        ρ          -                      ρ            a                                                ρ            c                    -                      ρ            a                              )        ×    100  where ρ is the density of the PET material; ρa is the density of pure amorphous PET material (1.333 g/cc); and ρc is the density of pure crystalline material (1.455 g/cc).
Container manufacturers use mechanical processing and thermal processing to increase the PET polymer crystallinity of a container. Mechanical processing involves orienting the amorphous material to achieve strain hardening. This processing commonly involves stretching a PET preform along a longitudinal axis and expanding the PET preform along a transverse or radial axis to form a PET container. The combination promotes what manufacturers define as biaxial orientation of the molecular structure in the container. Manufacturers of PET containers currently use mechanical processing to produce PET containers having approximately 20% crystallinity in the container's sidewall.
Thermal processing involves heating the material (either amorphous or semi-crystalline) to promote crystal growth. On amorphous material, thermal processing of PET material results in a spherulitic morphology that interferes with the transmission of light. In other words, the resulting crystalline material is opaque, and thus, generally undesirable. Used after mechanical processing, however, thermal processing results in higher crystallinity and excellent clarity for those portions of the container having biaxial molecular orientation. The thermal processing of an oriented PET container, which is known as heat setting, typically includes blow molding a PET preform against a mold heated to a temperature of approximately 250° F.-350° F. (approximately 121° C.-177° C.), and holding the blown container against the heated mold for approximately two (2) to five (5) seconds. Manufacturers of PET juice bottles, which must be hot-filled at approximately 185° F. (85° C.), currently use heat setting to produce PET bottles having an overall crystallinity in the range of approximately 25%-30%.
After being hot-filled, the heat-set containers are capped and allowed to reside at generally the filling temperature for approximately five (5) minutes at which point the container, along with the product, is then actively cooled prior to transferring to labeling, packaging, and shipping operations. The cooling reduces the volume of the liquid in the container. This product shrinkage phenomenon results in the creation of a vacuum within the container. Generally, vacuum pressures within the container range from 1-380 mm Hg less than atmospheric pressure (i.e., 759 mm Hg-380 mm Hg). If not controlled or otherwise accommodated, these vacuum pressures result in deformation of the container, which leads to either an aesthetically unacceptable container or one that is unstable. Hot-fillable plastic containers must provide sufficient flexure to compensate for the changes of pressure and temperature, while maintaining structural integrity and aesthetic appearance. Typically, the industry accommodates vacuum related pressures with sidewall structures or vacuum panels formed within the sidewall of the container. Such vacuum panels generally distort inwardly under vacuum pressures in a controlled manner to eliminate undesirable deformation.
While vacuum panels allow containers to withstand the rigors of a hot-fill procedure, the panels have limitations and drawbacks. First, vacuum panels formed within the sidewall of a container do not create a generally smooth glass-like appearance. Second, packagers often apply a wrap-around or sleeve label to the container over the vacuum panels. The appearance of these labels over the sidewall and vacuum panels is such that the label often becomes wrinkled and not smooth. Additionally, one grasping the container generally feels the vacuum panels beneath the label and often pushes the label into various panel crevasses and recesses.
External forces are applied to sealed containers as they are packaged and shipped. In some instances, adjacent containers bump into one another while traveling down a conveyor or during handling and shipping. In some examples, containers may define bumpers formed in the sidewall of the containers. Generally, the label area is recessed into the sidewall of the container resulting in outwardly oriented sections immediately above and below the recessed label area. These outwardly oriented sections are commonly referred to as bumpers. As such, bumpers typically form a raised land area defining an outermost dimension in cross section of the container.
The bumpers serve to protect the label from damage which may occur when two or more containers contact one another during handling, shipping and transporting. Traditionally, bumpers are strong, rigid structures designed to resist any distortion or denting when exposed to impact forces created by bottle-to-bottle contact or other external forces created during handling, shipping and transporting of the containers.
Such bumpers are designed to have the strength to withstand the rigors of bulk container filling, capping, labeling, transporting and distributing. However, excessive external impact forces may cause a bumper to collapse, thus losing its ability to provide label protection. Because bumpers are traditionally rigid structures, the collapse of a bumper often results in buckling, which causes permanent deformation in the form of permanent denting or creasing. This permanent deformation, in addition to failing to provide sufficient label protection, results in a container which is aesthetically undesirable to the consumer.
Bumpers may be adapted to absorb certain impact forces during packaging, shipping and transporting. In some cases, however, impact forces may cause the container to temporarily or permanently buckle or crease at the respective bumper.
Thus, there is a need for an improved lightweight container, which can accommodate vacuum pressures resulting from hot filling and absorb impact forces without buckling or creasing the container during packaging, handling and shipping.